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Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: Jimmy K » Fri Jan 28, 2011 7:06 pm

Would stiff-mounting this cannon noticeably increase energy transferred to the projectile? I was going to make a mount to accommodate the recoil but I'm thinking I could just do this as It would probably be easier.
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Unread postAuthor: Technician1002 » Fri Jan 28, 2011 8:56 pm

On an air cannon, the amount of energy lost due to recoil is related to how far the cannon moves back during the shot instead of pushing the projectile forward.

On my cannons launching most stuff, the cannon moves less than 3 inches before the projectile is ejected. You can see the amount of barrel movement in some of my High Speed video.

One sequence with the launcher on sawhorses shows the recoil is minimal.

This changes some when large heavy projectiles are launched. This tends to knock adults over if they don't lean into the shot.

Hmm, just checked my posted videos. I didn't post one showing the recoil. Sorry.
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Unread postAuthor: Jimmy K » Sun Feb 06, 2011 10:06 am

Alrighty.
So here are the final specifications:

- Roughly 20 gallon chamber (75.71 liters or 4620 cubic inches)
- 20 foot long 4" bore barrel (about 3016 cubic inches - C:B ratio is 1.53:1)
- Two parallel 3" Toro P-220 Irrigation valves (14.13 square inches of porting)

- Breech Loaded
- Trailer mounted (old boat trailer)
- Pneumatic barrel elevation

- On board compressor
- On board Generator


Any thoughts/concerns before I start welding?
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Unread postAuthor: Fnord » Sun Feb 06, 2011 4:13 pm

I'm not sure what your finished setup will look like, but just don't let the recoil flex the valves... it should be absorbed by the trailer directly.

Also, for this type of stuff, it's a good idea to go over your welding and assembly operations mentally several times;
more than once I've gotten in a hurry and welded on something that got in the way of parts I had planned to attach later.
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Unread postAuthor: Jimmy K » Sun Feb 06, 2011 6:04 pm

Ok. Makes sense. The way I have it drawn up there is no tension on the valves, nor should there be while firing. I hope.

And good point about the order of assembly; I'll have to watch out for that.
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Unread postAuthor: jackssmirkingrevenge » Mon Feb 07, 2011 1:18 am

Those are some pretty serious specifications, it's going to be one heck of a finished launcher.

I think the dual valves are slightly overkill, in the sense that the difference in performance will probably not be that noticable vs a single valve, but if you have them, go for it :)
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Unread postAuthor: Technician1002 » Mon Feb 07, 2011 2:21 am

Due to the issues of transonic flow in the valves (how air turns the corners at high speed) I have to disagree on the performance of one vs 2 valves. 2 valves will still have a fairly high loss even though the port area of the combined valves is slightly larger in area than the barrel. The dual valves will have a considerable pressure drop across them limiting flow to about 1/2 that of a burst disk with no bends between the valve and barrel.
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Unread postAuthor: velocity3x » Mon Feb 07, 2011 10:53 am

@jimmy
Any thought of using a muzzle brake? With 12.56 in^2 of barrel cross section, even low firing pressure can create a significant secondary recoil.
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Unread postAuthor: Jimmy K » Mon Feb 07, 2011 9:39 pm

I am kinda concerned about the recoil. Ill most likely end up adding a muzzle break if initial tests suggest explosive recoil force while within operating conditions.
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Unread postAuthor: jackssmirkingrevenge » Mon Feb 07, 2011 11:37 pm

Will the trailer be free to move? That would certainly help with avoiding damage to the launcher. Unless it's on the edge of a cliff.

Mounting the launcher itself on a sliding mount shouldn't be too big a deal either, add a few scrap parts from a motorbike suspension system and you're sailing.

Here's a good example Larda made for his terrific hybrid:

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Some gratuitous footage of German railway artillery showing the dual recoil principle in action, where the gun itself recoils in its mounting, while the mounting itself is free to slide back:

[youtube]http://www.youtube.com/watch?v=nAxySDG_jjs&feature=related[/youtube]

I note from your first post you want computer controlled elevation and traverse, in which case a moving mount (trailer in this case) would be a hindrance to accuracy and repeatability.
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Unread postAuthor: Gaderelguitarist » Tue Feb 08, 2011 12:00 am

I believe D_Hall's method of fighting recoil on Vera was to make it one hell of a solid cannon. The heavier the cannon, the easier the recoil forces will be received. Just mount everything solidly and put wheels on it if you're still concerned.
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Unread postAuthor: velocity3x » Tue Feb 08, 2011 12:13 am

Muzzle brake and solid / ridged mount to trailer frame works well for me.
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Unread postAuthor: Jimmy K » Tue Feb 08, 2011 3:43 pm

The trailer will most likely be hooked up to my truck, but I guess at this point I can change that too.

I don't think a sliding mount would be feasible, unfortunately, as there will be several points of attachment to the trailer.

What are the chances that the barrel (or the tee) would crack (it's PVC but will be supported) if fired from a stiff mount?
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Unread postAuthor: Technician1002 » Tue Feb 08, 2011 8:56 pm

For 4 inch, may I recommend ABS instead for the barrel? It is much less fragile. I have tested 4 inch cellular core drain pipe with 100 PSI pressure in it. All it will need is a little support to prevent barrel whip.
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Unread postAuthor: Jimmy K » Tue Feb 08, 2011 9:17 pm

I was given the 20 foot section of PVC and would like to use that first to minimize cost. Otherwise Id buy SCH 10 aluminum.
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